1. Field of the Invention
The present invention relates to a process for ion nitriding aluminum and aluminum alloys.
2. Description of the Related Art
The technologies of surface treatment of aluminum and aluminum alloys (hereinafter referred to as aluminum material) have been developed to remedy the low hardness and poor wear resistance of aluminum material. One of the technologies is the formation of an aluminum nitride layer on the surface of aluminum material. Aluminum nitride has several superior characteristics: thermal stability at very high temperatures, high hardness (Hv 1000 and above), high wear resistance, high thermal conductivity, and good insulation properties.
There have been proposed several processes for forming an aluminum nitride layer. According to one process (disclosed in Japanese Patent Laid-open No. 25963/1981), nitriding is accomplished by heating part of an aluminum material (to be treated) above a melting point of aluminum, thereby causing aluminum to react with nitrogen. A disadvantage of this melting process is that the aluminum material to be treated deforms upon melting and the resulting surface layer is a mixture of aluminum nitridge (AlN) and aluminum (Al), which has a hardness lower than Hv 200. Alternative processes include reactive sputtering and vacuum deposition. These processes, however, only provide an aluminum nitride layer which is attached to the base layer by mechanical force or intermolecular force and hence is poor in adhesion to the base layer. Moreover, they are not suitable for mass treatment and are expensive.
Under the circumstances, the present inventors filed applications for patent on "Process for ion nitriding of aluminum or an aluminum alloy and aparatus therefor" (U.S. Pat. No. 4,522,660/1985), "Process for ion nitriding aluminum or aluminum alloys" (U.S. Pat. No. 4,597,808 and Japanese Patent Application Laid-Open No. 202071/1987). The first one is characterized by placing a metal having a strong affinity for oxygen near the object to be treated in the ion nitriding apparatus so that the metal removes oxygen (inhibitor of ion nitriding) which enters the apparatus, thereby helping the formation of a good nitride layer on the object. The second one is characterized by activating the surface of the object to be treated by introducing a gas for nitriding into the sealed vessel, allowing discharge in the vessel prior to ion nitriding so that a good aluminum nitride layer is formed on the object. The third one is characterized by roughening the surface of the object to be treated to a roughness of 0.1 .mu.m and above (in terms of R.sub.2) prior to ion nitriding so that a good aluminum nitride layer is formed easily on the object. These technologies were successful with the formation of a nitride layer having good wear resistance and good adhesion on the surface of an aluminum material.
Nevertheless, these prior art technologies still have disadvantages. In the first case, it is possible to remove oxygen entering the ion nitriding apparatus, but it is impossible to remove oxides formed on the object for treatment. It has another disadvantage that it takes a longer time or the resulting nitride layer easily peels off if the nitridge layer is thicker than usual. In the second and third cases, the surface activating or roughening with a rare gas (such as argon) takes a long time.
In order to solve the above-mentioned problems encountered in the prior art technologies, the present inventors carried out a series of researches, which led to the present invention.